1. Field of the Invention:
This invention relates, in general, to communication receivers and, more specifically, to receivers for a distribution network power line carrier communication system.
2. Description of the Prior Art:
Generally, a distribution network communication system includes a central communication terminal which produces an outgoing communication signal intended for at least one remote communication terminal, usually located at a customer's premises. An injection point circuit, located at a substation or other convenient location, receives the outgoing communication signal through a telephone link or the like. The injection point circuit transmits a carrier signal, modulated by an information signal responsive to the received outgoing communication signal, over a utility's power distribution network. The remote communication terminal, on receiving the transmitted signal, performs the desired automated functions and, when appropriate, sends a response signal, in the form of a carrier signal modulated by an information signal, back to the central communication terminal. Receivers would, of course, be employed in the remote communication terminal and in the injection point circuit. More sophisticated communications systems requiring signal repeaters would use additional receivers. Irrespective of the remote device in which it is incorporated, the receiver is coupled to the power distribution network such that it receives the carrier signal; the carrier signal is then amplified, filtered and demodulated.
There are several techniques for modulating a carrier signal with an information signal so as to impress the information upon the carrier. In one such technique, phase shift keying modulation, PSK, the information signal shifts the instantaneous phase of the carrier between predetermined discrete values. Phase shift keying receivers capable of receiving, amplifying, and demodulating PSK signals are well known and understood in the prior art.
One prior art receiver technique for amplifying and demodulating a phase shift keyed modulated sinusoidal carrier signal transmitted on a distribution network power line communication system provides for initial high pass and low pass filtering followed by amplification with an automatic gain controlled amplifier. Excessive signal peaks are then attenuated by a clipper circuit and the signal passed through a bandpass filter. Following a second amplifier stage, again with automatic gain control, the signal is hard limited wherein the sinusoidal waveform is transformed to a square wave. Finally, the modulated carrier signal is demodulated.
The primary objection to this prior art receiver is its expense due to the large number of receiver stages utilized. For example, it requires two stages of amplification, both with automatic gain control. Since hundreds of receivers are used in a typical distribution network power line carrier communication system, the individual receiver cost must be kept low, consistent with proper performance, to make the overall communication system economical. Further, this receiver's noise performance is not satisfactory given the environment (i.e., a distribution power line network) in which it must operate.
The earliest prior art technique for coupling the above-discussed receiver to a distribution network power line is voltage coupling, i.e., by use of a capacitor. See, for example, U.S. Pat. No. 4,142,178, issued to Whyte et al., which is assigned to the assignee of the present invention. While successful for typical overhead substations with 4-6 feeders per bus, capacitive coupling has not performed satisfactorily for underground systems. Underground systems have greater load concentration at the substation bus, where it is not unusual to encounter at least twenty to twenty-five feeders on a single bus and possibly as many as forty-eight feeders in a large urban substation. The resultant driving point impedance may be as low as one ohm at the carrier frequency. This requires excessive power amplification at the injection point to provide sufficient signal amplitude at the most remote receiver location. In fact, it may require several kilowatts of signal power. Also, capacitive coupling is both a dangerous and expensive method of coupling the receiver to the distribution power line. Finally, these capacitive coupling techniques are strictly passive, i.e., no gain stages.
Another prior art coupling technique uses inductive coupling with fixed gain. See, for example, U.S. Pat. No. 4,016,429 issued to Vercellotti et. al. which is assigned to the assignee of the present invention. When high magnitude noise or carrier signals are encountered by this type of receiver, the electronics saturates thereby causing the lose of signal data. To overcome this objection manual gain control was added. Manual gain control is, however, both cumbersome and time consuming. Also, since noise varies with time and the distribution feeder choosen to carry the signal, manual gain adjustment still often results in saturation of the electronics or insufficient amplification. Use of an automatic gain control eliminates these manual gain adjustments.
One prior art automatic gain control technique uses a junction field effect transistor to control the gain of the amplification stage. See Electronics, Aug. 16, 1973, pages 99 through 100. Automatic gain control is accomplished by connecting the drain-source circuit of a field effect transistor between the amplifier input and ground; in cooperation with the amplifier feedback circuit, the drain-source resistance, which is dependent on the signal magnitude, controls the amplifier gain. While this gain control technique eliminates manual gain adjustment, it requires both positive and negative supply voltages, adding expense and space requirements to the receiver. Further, this automatic gain control has no provision for varying the signal level at which gain control will be triggered.
These disadvantages in the prior art are overcome by the present invention through its use of magnetic coupling; simplified, and less costly amplification and signal conditioning circuits; and an improved automatic gain control circuit design. These and other advantages are discussed in detail below in the Description of the Preferred Embodiment.